The development in recent years of copying devices such as full-color copiers has enhanced the quality of reproduction to a level at which one cannot distinguish the copy from the original with the naked eye. Such faithful reproductions can now be obtained inexpensively. However, we must consider that along with this benefit comes the increased risk of copiers being put to such illicit uses as counterfeiting money, negotiable securities, and other original documents whose reproduction is legally prohibited. To avert this danger, various devices have been developed to prevent copiers from being used for counterfeiting. One such device is the image processing device disclosed in Japanese Patent publication 2-210481, 2-210481 and an American patent application Ser. No. 08/321,651 filed on Oct. 11, 1994.
That prior art image processing device reads the original image placed on the glass of the copier by scanning the entire original four times. It is used in full-color digital copy machines. Let us assume that a bank note has been loaded into the copy machine. On the first scan, the device detects the rough location of watermark B on bank note A, the document whose forgery is to be prevented (See FIG. 68), on the assumption that the document on the glass is, in fact, a bank note.
On the second scan, the device detects, based on the location of watermark B, the exact position of the bill and the angle at which it is placed (i.e., the exact position coordinates of the bill on the glass). Since the length, size and shape of bank note A, the item to be detected, are known in advance, when the data exceed a given threshold value we convert them to binary values and extract edges C (hatched area) of bank note A. Based on the location of these edges, we obtain the coordinates of two of the bill's apices D, (x.sub.1, y.sub.1) and (x.sub.2, y.sub.2).
In this way we can obtain slope .theta.. On the third scan, the device calculates the position coordinates (x.sub.3, y.sub.3) of the red seal E stamped on the bill from the exact position of the bill obtained in the second scan (i.e., from the coordinates of apices D and angle .theta.). Based on these position coordinates, it extracts an image from the region in which seal E is placed, and it determines whether there is in fact a red seal on this item.
By scanning the item three times, the device is able to determine whether the document loaded in the copier is one which it would be illegal to copy (in this case, a bank note). If the device detects that a prohibited document such as a bank note has been loaded to be copied, it will perform its specified anti-counterfeiting operations on the fourth scan, such as making the entire display go black and preventing the copier from operating.
Since existing devices such as that described above require a number of scans to detect a bank note or other item which cannot be copied, arriving at a judgment is time-consuming. Furthermore, not all full-color copiers use a four-scan copying scheme like the one described above. Some scan the original three times, and others only once. The processing device described above cannot be used with these types of copiers. Another problem is that an extremely large memory capacity is needed merely to store data concerning one feature such as the size of paper money. Inevitably, then, only a limited number of kinds of money can be detected.
With the existing scheme, a pattern is selected from all the patterns on a document which may not be copied. This pattern must-have the appropriate number of features so that it can be clearly distinguished, and it must be a pattern which cannot be found on ordinary documents which may be copied. This pattern is matched against suspect patterns on the document to be copied. Even if the process is limited to a single country, there are an enormous number of documents which may not be copied. Attempting to detect all of them with certainty requires a huge memory capacity and lengthy processing time. Existing devices are unable to perform this processing in real time. If, after the copier is manufactured, a new document appears which may not be copied, the device will not be able to handle the document or even detect it. To detect the document, a new set of features must be selected and taught to the copier, which is a complicated procedure. If we are to include the documents of other countries, the problem becomes even more overwhelming.
Such a device requires three scans to determine whether the document loaded in the copier is a bank note or some other item which may not be copied. And if, for example, apex D of bank note A is folded or otherwise mutilated, the device will be unable to obtain the location of seal E. Consequently, it will be unable to determine whether the document being processed is actually bank note A.
One solution to this problem is shown in FIGS. 69 (A) and (B). Here guidelines F, consisting of thick solid or broken lines, are printed so as to enclose specified pattern E. We extract the interior of the region enclosed by guidelines F and determine whether this is the specified pattern. When guidelines F like those in the drawing are used, it becomes easier to separate the specified pattern from other image data. Using straight lines makes it easier to calculate slope, angles and coordinates of apices, so the pattern can be extracted more simply and accurately.
However, guidelines F are prominent enough (i.e., the feature is large enough) that they, and with them the pattern E which they designate, can easily be recognized. This makes it easier for a person to mutilate guidelines F or pattern E (by adding extra lines, erasing portions, and so on). Such mutilation may affect whether the pattern can be recognized (and so extracted), or even if the pattern is recognized, the device may be unable to determine whether it is the specified pattern.
When an existing scheme or the scheme described above using guidelines F is used, a pattern must be selected from the shapes found on the prohibited document which is well suited for a characteristic count, such as the red seal on a bank note. (A suitable pattern is one which can be conclusively distinguished and which will not be found on any ordinary document which may be copied.) Candidates are then matched against this pattern. Even if we limit the scope to a single country, there is a vast number of documents which may not be copied. To detect all of them reliably would require an extensive memory capacity and a significant amount of processing time. This would make it impossible to perform the copier processing in real time.
If, after the copier is manufactured, a new document appears which may not be copied, the device will not be able to handle the document or even detect it. To detect the document, a new set of features must be selected and taught to the copier, which is a complicated procedure. If we are to include the documents of other countries, the problem becomes even more overwhelming.